The present invention is directed to an optical reflectance gauge and method therefor and more specifically a gauge for measuring pulp optical properties.
Control of the chlorination stage of a bleaching plant is normally based on Kappa Number. That is, the chlorination stage is normally controlled to hold the amount of lignin residing on the pulp fibers (after chlorination and caustic extraction) constant. For such measurements, a standard transmission type gauge although desirable cannot be used because the pipe diameters necessary to pass this viscose material make the optical path so long that the signals are completely attenuated. This invention is essentially a single side transmission measurement in which the optical path is determined by the window separation.
Optical reflectance gauges have been developed in which a single window in the pulp pipe is used. Here a wide spectrum light source illuminates the passing pulp and one or more detectors provide electrical signals indicative of reflectance. These sensors, however, are limited by sensitivity to pulp consistency variation, pitch buildup, and boundary layer effects. Specifically, near the window a layer of stagnant material, such as black liquor, may occur. Also dirt or pitch may accumulate on the inside of the window to cause an erroneous color indication. In the case of simple reflection most of the returning optical signal is from the layer adjacent to the window.
The consistency of pulp has heretofore either been indirectly measured or directly measured by a mechanical impeller which is large and expensive. It has been discovered that consistency is related to pulp optical properties.